Improved Processing and Batch Time Reduction
Through Powder Liquid Dispersions
Stephen O’Rourke, Technical Director, Industrial
Abstract
The use of powder liquid dispersions in rubber compounds is well known, and many of
the processing, health and safety benefits are well documented. In this paper, the use of
powdered concentrates to reduce processing times is discussed.
High-viscosity liquids and resins, as well as high loading of low-viscosity esters and oils,
lend themselves to be dispersed on dry carriers and then added to an internal mixer or two-roll
mill. In general, these products are dispersions of liquids and semi-solids onto highly absorbent
carriers. The advantages of using these products include:
•
•
•
•
•
•
•
Elimination of the handling problems associated with high-viscosity liquids and semisolids by converting them to powder;
Elimination of the need for heated inventory storage of high-viscosity liquids or nonpouring liquids;
Reduction in batch preparation time;
Reduction in equipment clean-up time;
Improvement in batch incorporation time and raw material dispersion;
Reduction in the amount of costly materials wasted when viscous liquids or semisolids stick to the walls and bottoms of drum packages;
Improvement in package disposability by using fiber drums or paper bags as
standard packages.
These benefits can increase productivity and improve profitability. The percent active or
the amount of liquid that is absorbed is typically 72 percent.
Introduction
The addition of liquids to rubber compounds leads to problems in handling, incorporation
and dispersion. Despite these problems, many compounders incorporate various liquid
additives, ranging in form from low viscosity oils to viscous, tacky polymers. In large-volume
applications, the installation of storage tanks and metering pumps can offer a solution to these
handling problems, but in cases where rapid incorporation of the liquid and good dispersion are
required or where the engineering approach is not viable, the use of a powder liquid dispersion
(PLD) can eliminate many of the problems normally experienced.
In a PLD, the liquid is absorbed onto a suitable powder (or carrier) to form a product with
the appearance of a powder but which contains a high proportion of liquid. The amount of liquid
that can be carried depends on the liquid and the carrier used, but 72 percent is the normal
loading for many materials so that they can be easily handled without any dust being produced.
During mixing, the shear forces developed will break down the structure of the carrier so that the
liquid is gradually released into the mix, resulting in rapid dispersion.
Materials
Rubber compounding ingredients vary from ultra-high viscosity elastomers to water-thin
liquids with a myriad of powders, resins, accelerators, etc. and with varying degrees of
dispersibility. The mixing of all these ingredients together requires specific procedures regarding
mixing conditions such as temperature, time, speed of rotors, etc. To ensure quality, many raw
materials are modified for enhanced dispersion.
The following is a list of the materials that can be converted to powders.
•
•
•
•
•
•
•
•
•
•
•
•
•
Monomeric/Polymer ester plasticizers
Liquid polymers
− Nitrile
− Epichlorohydrin
− Polybutene
Process Oils
Coumarone-Indene Resins
Pine Tars
Petrolatums
Co-agents (monomers)
Peptizers
Chlorinated Paraffins
Accelerators
Antioxidants
Process Aids
Catalysts
Difficult solids can sometimes be handled more conveniently as a PDL. Materials
such as TEA, TAC and TAIC are normally supplied as a solid crystalline mass that must
be melted in order to be removed from the drums in which they are supplied. Some
waxes are so soft that if they are pelletized, they will rapidly agglomerate and form a
solid mass. If the materials are converted to a PLD, they will remain relatively stable and
will remain as powders even during hot weather. Petrolatums are much easier to handle
as a PLD versus the neat form. These products normally present no handling problems.
If the mix temperature is low, the time taken to melt and disperse the pellets may be
unacceptable. If sufficient time is not taken to achieve a complete melt, fragments of
undispersed pellets may survive the mixing process. By turning such materials into PLDs
they are incorporated into the mix in a much finer form, and melting and dispersion is
much more rapid.
Carriers
The choice of suitable carrier is critical when optimizing the performance of
PLDs. The carrier should give acceptable performance in the following areas.
•
•
•
•
It must be compatible with the compound and acceptable in the end
application.
It must not react or adversely affect the stability of the liquid component;
It must be cost effective.
It must release the liquid under the mixing conditions involved in the process.
If a proportion of the liquid is retained in the carrier, the efficiency of the PLD
will be reduced.
Amorphous Silica
Precipitated silicas are ideal carriers for use as PLDs. Precipitated silicas are
available in a variety of carrying capacities, particle sizes and dispersion characteristics.
Silica is a stable material that can be used in a variety of applications, for example, a
filler, carrier, thixotropic agent, clarifier and desiccant. These materials have a high
degree of porosity and can safely produce PLDs up to 72 percent liquid. These silicas
can sometimes react with other chemicals, whereas the use of a calcium silicate can be
a simple way of preventing such an interaction. There are grades of calcium silicate that
can carry 72 percent by weight of liquid content. The silicates tend to have more
reinforcing effect on a rubber compound than the precipitated silicas.
Fumed silicas can be used and have adequate carrying capacity but tend to be
dustier and are expensive.
Clays and precipitated calcium carbonate can be used to prepare PLDs with 30
and 40 percent activity, respectively. These tend not to be cost-effective because of the
increased amount of product needed for the finished end product.
In some cases, it is possible to use an existing filler from a formulation as a
carrier. This does not compromise the formulation and allows for a filler to be wetted,
which helps in reducing dusting and aids rapid incorporation of the filler.
Advantages of using PLDs
There are several reasons the use of powdered liquid concentrates can improve
production and profitability:
•
Elimination of handling issues associated with high-viscous liquids or semi-solids
Many of the plasticizers, resins, waxes and rubber chemicals require heating
to reduce their viscosity and make it possible for them to be weighed and dispersed.
Due to safety factors, the heating process requires that most of these chemicals be
heated slowly to avoid hot spots in the liquid and the possibility of thermal
degradation. It will generally take several hours or even several days to reach the
acceptable viscosity needed for dispensing. Once a material reaches a temperature
at which it can be processed, the equipment used needs to be capable of
maintaining this temperature, possibly for long periods.
•
Reduction in clean-up time
Unless liquid is metered directly into a suitable internal mixer, containers must
be provided for weighing and handling liquids and an appropriate method must be to
avoid increased spillage. If disposable containers are used, they will be
contaminated by the liquid and will need to be treated as special waste. The cost
associated with this type of waste is high and will increase as time goes on.
Reusable containers are an alternative but must be kept clean, covered and carefully
stored to prevent contamination. PLDs can be without these waste or clean-up
issues. An ideal method to eliminate all waste and clean-up is to purchase the PLDs
in low-melt bags.
•
Reduction in batch preparation time
The elimination of handling drums, pouring out liquids, heating and clean-up
will directly affect batch preparation time. Another possible time-saving measure is to
purchase the PLDs as preweighs. The combination of using a low-melt bag with a
pre-measured amount of PLD will provide large time savings in handling and
weighing. The added benefit of using preweighs to improve quality is well
documented.
•
Improvement in batch incorporation time and raw material dispersion
Mixing liquids of widely divergent viscosities together is a difficult operation.
Most of the plasticizers, oils, resins, and liquid polymers are magnitudes lower in
viscosity than the elastomers they are being mixed in. An everyday analogy is mixing
bread dough. If you try to mix all the oil into the flour at once, you tend to get a lumpy
lubricated mixture. To get an even distribution of the oil and have a complete mix, it
takes a great deal of work and time. If the oil or liquid is incorporated slowly, the
compound will have an even distribution. Table I compares the viscosities of several
liquid additives to elastomers.
A PLD subjected to the shear of a mixing process first forms a stiff paste, which
quickly incorporates. Further shear reduces the carrier structure, leading to a very
gradual release and, accordingly, the uneven softening that occurs with a liquid.
Low-viscosity plasticizers are easy to handle and not normally put onto a silica
carrier to improve handling or processing. The problem occurs when adding or injecting
these types of materials into an internal mixer; the viscosities of the liquid and the
polymer are vastly different, thus causing delay for full incorporation of the plasticizer in
the polymer matrix.
In this study, a comparison of DOP both neat and as a PLD was added to a nitrile
compound. Table II compares mixing times and energy consumption.
The results show that mixing times can be dramatically reduced. The batch
mixing time reduction can improve volume output and lower energy requirements. In an
actual factory case, this same data was applied to a similar compound that had a mix
time of 210 seconds. By switching to a PLD, the mix time was reduced by 90 seconds.
Because this compound was of significant volume, the producer was able to delay the
purchase of new mixing equipment.
•
Accurate weighing and improved quality
Compounders using SPC to control the quality of their products find that
replacing a liquid with a PLD improves product consistency. This had traditionally been
attributed to the improved accuracy of addition levels achieved with PLDs, but it is
probable that the improved dispersion achieved with PLDs also contributes to the
reduction in batch-to-batch variation.
Accurately weighing out a powder is a simpler operation than weighing a liquid.
Any small spillages of PLD can be swept up easily and will not coat and contaminate
scales or other equipment. These properties of clean handling also overcome the
problem of allowing for liquid retention in weighing containers. It is almost impossible in
practice to completely drain a liquid from a container. Normal practice is to make an
allowance for the expected retention when weighing out material, but the amount of
material retained can vary considerably. Achieving accurate addition levels becomes
difficult, as the operator has to judge when the container has drained sufficiently. Where
materials have been heated to reduce the viscosity, some cooling will occur between the
materials being weighed and being added to the mix. Viscosity will vary with
temperature, and using a standard time to drain the container will not give standard
retention of liquid. Scraping out the container will give lower retention figures but can be
time-consuming and increases the operator’s chance of coming into contact with the
liquid.
TABLE I
Viscosity of Various Liquid Additives to Elastomers
Materials
Dioctyl Phthalate (DOP)
Trioctyl Trimellitate (TOTM)
Process Oil
Polymeric Plasticizers
Liquid Nitrile
Polybutene
Coumarone – Indene Resin
Pine Tars
Coagents – TAC
Waxes
Viscosity Range, cps at 25°C
57
260
40-5000 SUS at 38°C
1,000–160,000
20,000–30,000
2,750–35,000
Solid
600–solid
Solid
Solid
TABLE II
Comparison of Mixing Times of Neat Liquid versus PLD
Plasticizer Level, PHR
Rheomix 600, 77 rpm, 93°C
Maximum Torque, m⋅g
Compound Temperature, °C
Maximum Energy, kJ
Dispersion Time, min.
DOP
30
PLD DOP
30
2080
108
64
10.0
2400
111
46
7.0